One form of fuel tank comprising a thermoplastic material, for installation in a motor vehicle, comprises a connecting portion extending around an opening in the tank wall, with the opening being closed by a holding cover which is fitted on to the connecting portion and held thereon by a fixing means for urging the holding cover in a direction towards the connecting portion. The holding cover is provided for holding for example conduits or hoses, filling level indicator means, a pump or the like. A sealing ring which may be in the form of an O-ring is interposed between the holding cover and the connecting portion to which the holding cover is secured. In a fuel tank of that general design configuration, as is to be found in German patent specification No 42 40 629, the arrangement is such that the holding cover lies exclusively on the resilient sealing ring in order in that way to be able to apply contact pressing forces of different magnitudes and compensate for any manufacturing tolerances that may be present. In that arrangement the resilient sealing ring is a structural component of the connection between the tank and the holding cover. That however suffers from the disadvantage that the sealing ring is required to perform two functions, namely a sealing function on the one hand and on the other hand the function of a structural member for the transmission of forces, with the result that a variation in the position of the sealing ring, which can be caused for example by swelling or creep movement of the material forming it, as may occur for example due to the action of fuel thereon, may possibly also at the same time result in the configuration of the connection being affected.
Reference may also be made to German laid-open application (DE-OS) No 31 35 982 disclosing a fuel tank of thermoplastic material for motor vehicles, with a filling connection portion or filler neck, and a connecting support which is mounted thereto, for receiving the closure cap of the filler. In that case a sealing ring is arranged between the filler neck and the connecting support portion which is supported against a contact surface of the filler neck by way of the elastic sealing ring. In this case also the aim of the invention is to make use of the elasticity of the sealing ring to compensate for tolerances which may be caused by virtue of dimensional fluctuations and different coefficients of expansion so that this design also essentially suffers from the disadvantages already described above in connection with the fuel tank of German specification No 42 40 629.